In many instances it can be desired to detect subterranean activity at a remote location. Ground penetrating radar systems can be used to map subterranean regions of the Earth's crust. Generally, the subterranean region is located underground within approximately the upper 10 km of the Earth's crust and may include relatively stationary and non-stationary subterranean formations. Information about these formations may be of particular interest since faults, earthquakes, and fluid migrations at these levels may be more likely to impact structures and life on the surface of the Earth.
Typical ground penetrating radar systems can detect activity directly below the radar installation, by forming a vertical field of regard with respect to the target area. Therefore such systems generally necessitate vertical access through the ground to the subterranean target image areas, as well as access to the above-ground terrain. However, in many circumstances characteristics of the territory prevent vertical or direct above-ground access. For example, vertical access directly above the target area may be challenging due to unstable or impenetrable rock or soil formations. Similarly, excavation of the terrain directly above the target area may be prohibited due to, for example, conservation efforts or inhabitation. Furthermore, access to territory may be forbidden by domestic or international laws.
Some seismic systems and acoustic sensing systems can be used to detect and provide information about relatively large geological formations, such as oil and gas deposits, without the need for vertical or direct above-ground access to the target area. However, these systems are unable to provide the high resolution imaging needed for detailed mapping of the subterranean target area.